As is known, MRI (Magnetic Resonance Imaging) is a technique used for getting images of a body without the use of X-rays. In MRI, the patient is placed within a rapidly changing magnetic field created by the MRI device. Radio waves are simultaneously transmitted to the patient, and images of the interior of the body are obtained by means of a computer analysis of the radio waves received from the patient.
During the imaging, the condition of the patient is constantly monitored. This includes the monitoring of the ECG signal of the patient, for example. One drawback related to the monitoring is that the intense magnetic fields of the imaging device induce interference with the ECG signal of the patient. This interference, which is termed here an artifact, is short-termed and strong, and appears very frequently.
FIGS. 1a and 1b show a typical example of the narrow and strong interference peaks caused to the ECG signal by MRI measurement. FIG. 1a illustrates a sequence of the narrow interference pulses superimposed on the ECG signal, while FIG. 1b illustrates the shape and length of one interference pulse. The artifact is usually rather similar for each MR image acquired and consists of a few short peaks. The shape of an interference peak is roughly box-like, with a relatively constant period between the steepest rise and fall edges.
The prior art methods intended for suppressing the above-described artifacts are based on either limiting the rise and fall speed of the ECG signal or freezing the signal during the interference periods. In the following these prior art methods are discussed briefly.
A natural way of detecting the MRI gradient peaks is to monitor the rise and/or fall speed, i.e. the slew rate, of the ECG signal. A solution based on a slew rate limiter is disclosed in U.S. Pat. No. 4,991,580. This patent presents a method whereby the ECG signal is supplied to the input of a slew rate limiter (SRL) circuit, which limits the slew rate of the ECG to a pre-selected maximum value slightly greater than a typical maximum slew rate in an ECG signal.
Another approach for eliminating MRI induced artifacts superimposed on an ECG signal is presented in U.S. Pat. No. 5,038,785. This patent describes a cardiac monitor in connection with an MRI device, the monitor to include a comparator which compares each wave form received from the ECG electrodes with the properties of a cardiac signal, such as the slope. When the comparator detects interference in the ECG signal, it gates a track and hold circuit. The track and hold circuit passes the ECG signal, except when gated by the comparator. When gated by the comparator, the track and hold circuit continues to supply the same output voltage it had at the beginning of the gating period, i.e. it freezes the ECG signal for the period of interference.
U.S. Pat. No. 4,243,045 presents a method for the suppression of narrow interference pulse peaks and of interference hum in a useful signal. The method is intended particularly for suppressing the pacemaker pulses and the power supply hum superimposed on an ECG signal. The narrow pulses caused by the pacemaker are blanked out by keeping the signal constant for the blanking-out period, and the power supply hum is eliminated by an interference hum filter. The filtered-out interference hum component is opposingly superimposed on the original signal to compensate for the interference hum.
The prior art methods based on freezing the signal entail the fundamental drawback of not being able to take into account signal changes occurring during the freezing period. The methods based on limitation of the slew rate, in turn, involve the fundamental drawback that some of the interference remains in the useful signal, as the methods limit only the size of the interference. For these reasons, the prior art methods may leave errors of substantial magnitudes affecting the useful signal.
It is the objective of the invention to improve the accuracy of the methods described above and to bring about a new and efficient solution for suppressing or eliminating artifacts from a useful signal, particularly MRI-induced artifacts from an ECG signal.